1. Field of the Invention
The present invention relates generally to moisture sensors, and more particularly to optical fiber moisture sensors and methods for manufacturing the same.
2. Related Art
The monitoring and controlling of gas phase water content is required in many fields. Such fields include daily air quality management, weather broadcasting, agricultural activity, industrial process control in tobacco production, food processing, oil refinement, metal processing, high purity gas preparation, semiconductor production, etc.
In such varying fields, different applications require different sensors to monitor water concentration in a specific gas environment. For example, in a tobacco production line, a humid air environment (i.e., relative humidity is equal to 60% to 80%) is required in order to preserve the quality of the tobacco leaves. In such an application, a sensor with sensitivity for detecting water content of x % is appropriate and sufficient. However, in a semiconductor production line, water content in the gas stream needs to be controlled in the sub-part-per-billion level. Such an application requires a technique with very high sensitivity to monitor water content in the gas phase. An example of a sensor implementing such a technique (e.g., the Cavity Ring-Down Spectroscopy (CRDS) technique) is available from Tiger Optics, LLC of Warrington, Pa.
Present techniques for monitoring moisture include those that are based on the change of some electric property as detected by a specially-designed resistor or capacitor as described in U.S. Pat. No. 5,040,090 issued to Birkle et al.; and Raj et al., “Sensors and Actuators,” B: Chemical, B81(2–3), 229–236 (2002). Sensors based on these techniques are used in such applications as air quality management, food process control, soil analysis, etc. Because metal electrodes are used in the fabrication of such electric property based sensors, however, they normally have problems in applications involving corrosive environments where a high temperature gas stream, a gas stream of high humidity or a gas stream containing acidic species (e.g., SO2, NOx, HCl) is present. Further, these electric property based sensors are also susceptible to electromagnetic noise, which limits the applications in which they can be used.
An alternative to electric property based sensors is optical fiber moisture sensors based on end-point membrane light absorption, evanescent wave optical absorption, or evanescent wave excited fluorescence. These optical fiber sensors have several advantages over electric property based moisture sensors. These advantages include smaller size, easier and cheaper fabrication, lower operating costs, and immunity to electromagnetic noise. In addition, distributed optical fiber sensors can be made in a single fiber to sense moisture in different locations at the same time.
U.S. Pat. No. 4,634,856 issued to Kirkham describes a light absorption based optical fiber sensor where a reflective target is attached on one end of an optical fiber. The reflectance of the target depends on the content of moisture in gas phase surrounding the target. Kirkham also describes a moisture sensor based on the change of reflective index of the clad of an optical fiber.
U.S. Pat. No. 5,319,975 issued to Pederson describes a fluorescence based optical fiber sensor for moisture sensing. The transducer of this sensor is a reagent-trapped membrane that is attached to one end of an optical fiber. A monochromatic light beam, which is obtained by passing light from a high intensive lamp through a monochromator, is fed into the second end of the fiber to excite the fluorescence of the reagent trapped in the membrane. A second optical fiber is used to collect the fluorescence from the membrane. When the membrane is exposed to humid air the fluorescence is quenched. This sensor is described as being able to detect moisture to less than 10% relative humidity.
U.S. Pat. No. 4,894,532 issued to Peterson et al. describes a plasma polymerization method to coat polymer from a gas mixture of hexamethyldisiloxane and ammonia on the surface of an optical fiber core. While the characteristics of the coated polymer are not described in Peterson et al., the polymer is described as absorbing light guided through the optical fiber on which the polymer was coated. The polymer coating process is complex and time consuming. Peterson et al. describes a process where eleven hours of plasma discharge is required to coat the polymer onto the surface of a fiber core.
Optical fiber sensors based on indicators trapped in polymer coatings have also been reported. See, e.g., Hypszer et al., SPIE, vol. 3054, 145–150 (1997); and Jindal et al., Optical Engineering, vol. 41, 1093–1096 (2002). CoCl2 is the most frequently used indicator in these sensors. CoCl2 can absorb light with a large peak-absorption at around 690 nm and a minor peak-absorption at around 550 nm. This compound can form a complex with water molecules when exposed to humid air. The complex absorbs light with peak absorption at around 550 nm. CoCl2 is usually doped in a polymer, such as polyvinylalcohol (PVA), polymethylmethacrylate (PMMA), gelatin, polyvinylpyrrolidone, etc.
To construct the transducer of an optical fiber sensor the CoCl2 doped polymer is coated on top of an optical fiber core. A (He—Ne) laser is usually used as a light source to probe the absorption signal. As observed in Hypszer et al., a CoCl2 doped optical fiber moisture sensor usually cannot obtain a linear calibration for a wide humidity range. It is the concentration of CoCl2 complex with water, Co[H2O]6Cl2, which has linear relationship with water content in gas phase. However, the absorption signal of Co[H2O]6Cl2 cannot be used in these sensors because CoCl2 also absorbs light at the peak absorption wavelength (i.e., 550 nm) of the complex. These sensors usually monitor the absorption signal of CoCl2 at 630 nm. This signal, however, is not proportional to water molecule concentration in gas phase.
Optical fiber sensors with organic polymer as a coating material are also limited in the applications for which they can be utilized due to the stability of organic polymer when exposed to severe environment (e.g., high temperature, corrosive gas, intensive ultraviolet light radiation, etc.).
Therefore, given the above, what is needed is an optical fiber moisture sensor that overcomes the above-described limitations and is capable of being used in varying applications.